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250 oz-in double-shaft step motor  with 36" cable (left), none (right)

Note 1: As motors spin faster, more voltage is required if you want to maintain rated current (that is what chopper driver circuits  do).  The effective resistance (called impedance, Z) increases with speed due to the inductance in the motor windings.  Impedance is the sum of the motor resistance (Rp) and reactance.  Reactance = 2 x Pi x f x Lp, where f is the frequency in pulses per second.  So for example, you are cutting plastic on a Sherline mill in half-step mode with a unipolar driver and motor at 3 in/min.  Then: f = 3 in/min * 1 min/60sec * 20 thread/in * 1 rev/thread * 400 steps/rev = 400 steps/sec.  Reactance = 2 x 3.14 * 400 * 2/1000 = 5 ohm.  So the impedance would be 5 + 1.11 = 6.11 ohm.  The voltage required to maintain the rated current would be 2.5 * 6.11 = 15 volts.  But you would burn out the motor when it stopped moving if you left it at that voltage.

Motor Data: All values typical at 40 deg. C. unless otherwise noted		Units	Bipolar 4-Lead
Ic	Rated continuous current/phase	Amps	2.0
V	Holding voltage, ie. motor not moving (surges up to 65V can be applied to reach rated current quickly). See Note 1.	Volts	4.7
Rp	Resistance: 24 deg +/- 10%	Ohms	2.33
Lp	Phase Inductance +/- 20%	mH	11.5
Th	Holding Torque (2-Phase-On)	oz-in (N-m)	250 (1.74)
D	Degrees per step		1.8
W	Weight	lb	2.3
F	Frame Size	NEMA	23

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